• New research suggests that our sleep cycles and activity patterns may influence our risk of type 2 diabetes and heart disease.
  • People who prefer to stay up late, night owls, have a reduced ability to utilise fat as energy, increasing their chance of developing metabolic diseases.
  • On the other hand, early risers, early birds, rely more on using fat as energy, have increased insulin sensitivity, and are typically more active during the day than their night-time counterparts.

American researchers from Rutgers University, New Jersey, have found that people who are active later in the day have an impeded ability to burn fat for energy. Being unable to use fat for energy efficiently can lead to a build-up of excess fat within the body and increases the risk of developing type 2 diabetes and heart disease.

Compared to night owls, people who are active in the morning utilise more fat as energy when resting and exercising. They also had increased insulin sensitivity, requiring less of the hormone to regulate their blood glucose levels.

Senior author Professor Steven Malin, Rutgers University, New Jersey, USA said: “The differences in fat metabolism between ‘early birds’ and ‘night owls’ shows that our body’s circadian rhythm (wake/sleep cycle) could affect how our bodies use insulin.

“A sensitive or impaired ability to respond to the insulin hormone has major implications for our health. This observation advances our understanding of how our body’s circadian rhythms impact our health.”

To explore the association between circadian rhythms and our health, the researchers split participants into two groups – early and late – based on their ‘chronotype’ or their natural propensity for sleeping and being active at different times of the day. They were then able to assess the participants’ body mass and body composition using advanced imaging.

Both groups were monitored over a week to assess their daily activity patterns and were also provided with a calorie and nutrition-controlled diet. All the participants fasted overnight to minimise the dietary influence on the results.

After being tested at rest, both groups took part in two 15-minute exercise sessions on the treadmill, one at moderate intensity and the other at high intensity.

It was found that the early bird group utilise more fat for energy when resting and during exercise than the night owls, as well as having a higher insulin sensitivity.

The night owls were more insulin resistant, requiring more of the hormone to lower their blood glucose. At the same time, their bodies also preferred using carbohydrates as a source of energy over fat.

While the reason behind this metabolic shift requires further investigation, it indicates a greater risk of metabolic diseases, such as type 2 diabetes and heart disease, in people with a late chronotype.

Professor Malin concluded: “Further research is needed to examine the link between chronotype, exercise and metabolic adaptation to identify whether exercising earlier in the day has greater health benefits.”

This study was originally published in the journal Experimental Physiology.

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